A number of microorganisms are known which are beneficial in plants. For example, vesicular arbuscular mycorrhizal (VAM) fungi are microorganisms found in soil which enter into a beneficial symbiotic relationship with many plants. These fungi aid the host plant in obtaining nutrients, particularly phosphorous, from the soil, and appear to confer resistance to plant pathogens. Many of the crops capable of forming beneficial symbiotic relationships with VAM fungi are susceptible to common pathogenic fungi found in the rhizosphere.
A typical way of treating soil infected with pathogenic fungi or other pathogens is to treat the field in its entirety with a fungicide or fumigants. This procedure is most relevant when it is unclear that the soil organisms present are appropriate for the intended crop, such as in areas of crop rotation, reclamation of nonagricultural land, e.g., pollution sites, mine spills or sand dunes and in other areas where rapid establishment of crops is needed. The most common techniques of fumigation, with agents such as methyl bromide, eliminate beneficial microorganisms as well as the plant pathogens. Therefore, reinoculation with beneficial microorganisms is of special importance in aiding recolonization and crop growth as well as preventing reestablishment of undesired microbes.
Using VAM infection as an illustration, various methods of effecting beneficial infection by VAM fungi have been disclosed. For example, Nemec, Trop Agric (Trinidad) (1983) 60:97-101 describes the inoculation of citrus seedlings by dipping plant roots in an inoculum comprised of VAM fungi and a sticking agent, and the effect of various fumigants and fungicides on VAM fungi has been studied. See, e.g., Menge, Phytopathology (1982) 72:1125-1132; Nemec, Can J Bot (1980) 58:522-526. In addition, Johnson, C. R. et al., J Environ Hort (1985) 3:166-168 report the use of hydrophilic polymers as carriers for VAM inocula. In their experiments, four methods of inoculation were used: mixing the inoculum into the soil of potted plants; placing the inoculum directly beneath the root system of the cutting; dipping the root system into a slurry of "Terrasorb", a commercially available polymer manufactured by Industrial Services International, Bradenton, Fla., at 1.5 g/100 mL water; and dipping the root system into a slurry of Viterra plant gel, manufactured by Nepera Chemical Company, Harriman, N.Y., at 1 g/100 mL water. The particular polymers and dipping protocols used in the Johnson article are said to have resulted in lower inoculation levels than the alternative processes known in the art. In addition, the article by Beswetherick, J. T. et al., Trans Br Mycol Soc (1987) 89:603-605 describes the inoculation of roots with VAM by sandwiching the roots between squares of cellophane to which the fungal mycelium has been attached and references a number of other techniques for inoculation.
Hwang, Plant Disease (1988) 72:448-452, describes the treatment of seeds with the fungicide metalaxyl followed by planting in soil inoculated with a VAM fungus. Groth et al., Plant Disease (1983) 67:1377-1378, describes the effect on plant growth of using soil treated with both metalaxyl and VAM fungi. Strider, Plant Disease Reporter (1977) 61:746-748, describes the use of benomyl in a root dip to help control Rhizoctonia root rot. German Democratic Republic Patent No. 128,396 (1977), is directed to method of controlling root diseases of cereals by coating the seeds with a nutrient substrate for antagonistic microflora and a fungicide. Leong, Ann Rev Phytopathol (1986) 24:187-209, is a review of the role of siderophores in the biocontrol of plant pathogens. Melero-Vara et al., Plant Disease (1982) 66:132-135, is directed to a seed dressing containing metalaxyl to control downy mildew.
It is also known to coat the roots of seedlings during transplantation with a polymeric material to, for example, prevent desiccation. See, e.g., Hamilton et al., Tobacco International (1982) 184:88-91; Federal Republic of Germany Patent No. 1,945,110 (1970); Japanese Patent No. 57/083230 (1982); Japanese Patent No. 54/117734 (1979). In addition, U.S. Pat. Nos. 4,434,231 and 4,755,468, both to Jung, disclose crosslinking of polymer vehicles used to coat root systems by various means. Crosslinking, however, may suppress spore germination and infection.
There are currently at least four known methods of enhancing the infection of plant roots with beneficial microbial cultures also known as live soil inoculants. These methods include broadcasting, banding, layering and root coating. The broadcasting methodology relies on a "shotgun" approach of inoculation whereby pellets of inocula are disseminated over large areas of soil with no control over how much of the inocula will reach the rhizosphere and ultimately lead to root infection. Banding involves a methodology whereby lines or "bands" of inocula are alternated with rows of plants. Inoculation of the roots with the microbes and the banded inocula takes place as the plant roots grow and spread through the lines or bands containing the inocula. The method does not provide any control with respect to root infection and is merely facilitated by the positioning of o the inocula and the plants in bands or lines. Layering is a technique whereby layers of inocula are placed beneath the roots of seedlings. This methodology works in a manner similar to that of "banding" in that the roots of the plants become infected by the microbes as the roots grow downward and through the layer of inocula. Thus, layering is similar to banding in that it does not control the efficiency of root infection. Lastly, root coating is a technique which increases the efficiency of the delivery of inocula to the roots by placing the roots in close contact with the inocula via coating. However, the methodology requires that precautions against root damage and contamination be taken, thus, increasing the level of effort required to carry out the procedure.
As indicated above, there are techniques available which attempt to enhance the degree of infection of plant roots by beneficial microorganisms. However, such methods suffer from a number of disadvantages. Accordingly, there continues to exist a need for an improved economical and efficient method for enhancing the degree of root infection with beneficial microbes. The present invention provides such a method.